1. Field of the Invention
The present invention relates to apparatus for reproducing information recorded on a recording medium such as an optical disk and, more particularly, to an apparatus for reproducing recording information recorded on a main track of a recording medium while canceling crosstalk from adjacent tracks included in the signal read from the main track.
2. Description of the Prior Art
Recording of information on optical disks at higher densities has been pursued by narrowing the track pitch. However, in using the method wherein a track pitch is narrowed, the track pitch is limited by the diameter of a laser beam focused on the disk. A signal from a main track is undesirably accompanied by signals from tracks adjacent thereto when the track pitch is narrowed without changing the laser beam diameter. Thus, a problem has arisen in that an increase in crosstalk from the adjacent tracks reduces the signal-to-noise ratio, making it impossible to reproduce a recorded signal accurately.
Under such circumstances, control has been employed in an apparatus for reproducing information recorded on a recording medium utilizing, for example, an optical pick-up having three beams for reading. The main track is irradiated with a main beam under tracking control to read a signal associated with the main track, and a crosstalk signal is produced based on an output of received light which is two sub-beams projected on and reflected by respective tracks adjacent to the main track, i.e., inside and outside the main track. The crosstalk signal is subtracted from the signal read from the main track to cancel the crosstalk, thereby reducing the effect of the crosstalk from each of the adjacent tracks, i.e., inside and outside the main track to be reproduced.
FIG. 9 shows an example of a signal block diagram for a crosstalk reducing circuit in the above-described conventional apparatus. The main track is irradiated with the main beam to obtain a signal associated with the main track (hereinafter referred to as "CENT signal") from the light returned therefrom. Coefficient control circuits 2 and 5 detect crosstalk components included in the CENT signal from the adjacent tracks and set filter coefficients for variable filters 1 and 4, depending on the quantities of residual crosstalk.
The variable filters 1 and 4 have attenuation characteristics that are in accordance with coefficients set by the coefficient control circuits 2 and 5, respectively. The signal from the adjacent track inside the main track (hereinafter referred to as "IN signal") and the signal from the adjacent track outside the main track (hereinafter referred to as "OUT signal") a re converted into crosstalk signals from the adjacent tracks by being passed through the variable filters 1 and 4, respectively.
A subtractor 14 subtracts the crosstalk signals from the CENT signal to produce a main track signal in which the effect of the crosstalk from the adjacent tracks has been reduced (hereinafter referred to as "CENT' signal"). The coefficient control circuits detect any signal components from the adjacent tracks remaining in the CENT' signal and calculates more appropriate coefficients in accordance with the quantities of the signal components of the adjacent tracks to set the filter coefficients of the variable filters 1 and 4 again.
FIGS. 11B through 11E show signal waveforms at various parts of the crosstalk reducing circuit in the conventional apparatus shown in FIG. 9 obtained from the pit pattern shown in FIG. 11A. FIG. 11B shows a waveform of the IN signal from the inner adjacent track, and FIG. 11C shows a waveform of a signal output by the variable filter 1. FIG. 11D shows a waveform of the CENT signal which is inputted to the crosstalk reducing circuit, and FIG. 11E shows a waveform of the CENT' signal which is inputted to a decoder 7.
As described above, the crosstalk reducing circuit of FIG. 9 removes crosstalk signals from the adjacent tracks inside and outside the main track from the CENT signal to generate the CENT' signal which is inputted to the decoder 7. The decoder 7 converts the inputted CENT' signal into a binary form to output it as a reproduction digital signal.
Meanwhile, the effect of the crosstalk from the adjacent tracks on the main track varies in its degree depending on the state of the main track (i.e., whether there is a pit or mirror).
FIG. 10 shows how the effect of the crosstalk from the adjacent tracks varies in its degree depending on the state of the main track, i.e., depending on whether there is a pit or mirror. Specifically, FIG. 10 shows variation in the quantity of the returned main beam that varies depending on the pit patterns of the main and adjacent tracks.
Referring to FIG. 10, when the region A is irradiated by the main beam, i.e., when there is a mirror on the main track and there is a mirror on both of the inner and outer adjacent tracks, the quantity of the main beam reflected is at the level indicated by "a". When the region B is irradiated by the main beam, i.e., when there is a mirror on the main track, a pit on the inner adjacent track and a mirror on the outer adjacent track, the quantity of the main beam returned is affected by crosstalk from the inner pit and is at the level indicated by "b,"which is lower than that indicated by "a".
Further, when the region C is irradiated by the main beam, i.e., when there is a mirror on the main track and there is a pit on both of the inner and outer adjacent tracks, the quantity of the main beam reflected is affected by crosstalk from each of the inner and outer pits and is at the level indicated by "c,"which is lower than that indicated by "b".
It is apparent from the regions described above that when the region on the main track irradiated by the main beam is a mirror, the quantity of the reflected main beam is greatly reduced by the presence of a pit on an adjacent track. Especially, in the case where there is a pit on both of the inner and outer adjacent tracks (the case of the region C), the quantity of the reflected beam available is only substantially one-half of that available when there is no effect of the adjacent tracks (the case of the region A).
Referring to FIG. 10, when the region D is irradiated by the main beam, i.e., when there is a pit on the main track and there is a mirror on both of the inner and outer adjacent tracks, the quantity of the main beam reflected is at the level indicated by "d". When the region E is irradiated by the main beam, i.e., when there is a pit on the main track, a pit on the inner adjacent track and a mirror on the outer adjacent track, the quantity of the main beam returned is affected by crosstalk from the inner pit and is at the level indicated by "e,"which is slightly lower than that indicated by "d". Further, when the region F is irradiated by the main beam, i.e., when there is a pit on the main track and there is a pit on both of the inner and outer adjacent tracks, the quantity of the main beam reflected is affected by crosstalk from each of the inner and outer pits and is at the level indicated by "f,"which is almost the same as that indicated by "e".
It is apparent from the above description that when the region on the main track irradiated by the main beam is a pit, the quantity of the reflected main beam is also affected by a pit on an adjacent track but to a degree less than when there is a mirror on the main track. That is, the effect of the adjacent tracks on the main track is greater when there is a mirror on the main track and smaller when there is a pit on the main track. Thus, the effect of the adjacent tracks varies depending on the state of the main track.
Referring to the signal waveform diagram in FIG. 11D, crosstalk components (the shaded regions in FIG. 11D) from a pit on an adjacent track (the inner track in FIG. 11A) is at a lower level when there is a pit on the main track and at a higher level when there is a mirror on the main track.
Therefore, when the coefficients of the variable filters are kept constant irrespective of the state of the main track, as shown in FIG. 11E, cancellation performed on the CENT' signal obtained by removing crosstalk signals will be excessive when there is a pit on the main track and will be conversely insufficient when there is a mirror. As a result, the effect of the adjacent track will still remain as indicated by the shaded regions in FIG. 11E. This equally applies to subtraction of a crosstalk signal that is obtained in accordance with the quantity of crosstalk from the outer adjacent track.
The effect of crosstalk from the adjacent tracks varies in its degree with the variation of signal levels that depends on whether there is a pit or mirror on the main track. It has been difficult for conventional apparatus for reproducing information recorded on a recording medium to properly cancel crosstalk from each of the tracks adjacent to the main track because the cancellation becomes excessive or insufficient.